The present invention relates to hydraulic brake systems and more particularly relates to anti-lock brake systems.
A brake system of this type is known from German Patent No. 4,025,859 A1, for example. In this known brake system, the brake circuit is divided into a front axle brake circuit and a rear axle brake circuit, wherein one brake circuit is assigned to the nondriven axle and the other brake circuit is assigned to the driven axle. The brake circuit of the driven axle is equipped with a traction control device. A traction control represents an active braking maneuver because the braking process is initiated without vehicle operator actuation of the brake pedal. Other types of active braking maneuvers are braking processes for controlling the yaw moment, for example, without participation of the driver or in addition to a pedal-actuated braking maneuver.
During an anti-lock braking maneuver, the known brake system operates in accordance with the recirculation principle, wherein the brake system is equipped with a precharging pump for the recirculating pump. The suction side of the precharging pump is connected to the reservoir of the brake system, and the pressure side is connected to the suction side of the recirculating pump via a hydraulically actuated on-off valve. Once the delivery pressure of the recirculating pump reaches the maximum pressure of the brake system during a traction control maneuver, the conveyed pressure medium flows through a pressure limiting valve and into the control line for the hydraulically actuated on-off valve arranged between the precharging pump and the recirculating pump. During the subsequent traction control, no precharging volume is advanced to the recirculating pump until the pressure in the control line has decreased to such a level that the on-off valve opens again. The purpose of this measure is that one attempts to prevent the unnecessary conveyance of the pressure medium through the recirculating pump. The recirculating pump should only generate a pressure upon demand. Consequently, excess pressure medium is not discharged into the reservoir of the brake system via a pressure control valve, but the suction side of the recirculating pump is blocked until the delivery of the pressure medium is required again.
A precharging pressure source represents one of the possible pressure sources for at least temporarily building up an increased pressure on the suction side of the recirculating pump. Other options for precharging a regenerative or nonregenerative recirculating pump are the utilization of an active vacuum brake booster, an additional pressure reservoir or, for example, a precharging device arranged between the reservoir and the main cylinder. In the latter instance, the volume flowing into the brake line through the main cylinder is used for precharging the recirculating pump when the brake pedal is not actuated.
It is also possible for a pressure source other than a precharging device to build up the pressure on the suction side of the recirculating pump. Brake systems, the second suction line of which branches off the brake line above a block valve, are only able to isolate the main cylinder pressure from the suction side of the recirculating pump if the second suction line is blocked. However, this only takes place after a pedal-actuated braking maneuver has been detected in the active braking mode, i.e., once the main cylinder pressure is able to propagate to the suction side of the recirculating pump.
In all aforementioned constructions, the recirculating pump is highly stressed and consequently subjected to premature wear by the precharging device in the active braking mode or by the pressure acting upon the recirculating pump from another pressure source. In extreme instances, large leaks can lead to the failure of the electric motors for the pumps. However, a submersible design of the electric motor would be associated with significant expenditures.
The present invention is based on the objective of developing a hydraulic brake system in which the recirculating pump is subjected to less wear.
The basic principle of the invention consists of limiting the pressure on the suction side of the recirculating pump to the required level. With respect to blocking the pressure medium supply, the delivery pressure of the recirculating pump is no longer decisive, but rather the pressure on the suction side of said recirculating pump.
In order to limit the pressure on the suction side of the pump, a hydraulically actuated valve is preferably utilized because a valve of this type requires no separate control logic and no electric lines. Such a valve need not contain any effective surfaces for the pressure medium in the valve opening direction because the output pressure which acts in the closing direction is automatically built up due to the activity of the recirculating pump such that the valve is opened.